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Let $K$ be an ellipse specified in terms of:

a given straight line $D$
a given point $F$
a given constant $e$ such that $0 < e < 1$

where $K$ is the locus of points $P$ such that the distance $p$ from $P$ to $D$ and the distance $q$ from $P$ to $F$ are related by the condition:

$q = e p$

The constant $e$ is known as the eccentricity of the ellipse.

Also denoted as

Some sources use $\epsilon$ for the eccentricity of a conic section.